A vast art now exists which relates to the production of aqueous dispersions and emulsions of thermoplastic resins from the corresponding monomer or monomers by the methods of emulsion polymerization. These polymerizations are customarily performed in the presence of a considerable volume of water which may contain colloidal protectors or stabilizers, emulsifying agents of various kinds, free radical polymerization initiators, activators and promoters added to modify the action of the initiator, chain transfer agents for regulating molecular weight, chelating agents to intercept and neutralize the effect of metallic ions, as well as certain organic solvents to lower the freezing point of the system and otherwise modify the polymerization. Many of the thermoplastic polymers produced in this way by dispersion or emulsion polymerization can also be converted to powders by such processes as coagulation, spray drying, etc.
In general, the design of a useful dispersion or emulsion polymerization system poses two different but interrelated problems:
(A) THE POLYMERIZATION ITSELF, WHICH, AS A PROCESS TAKING PLACE IN TWO OR MORE DISCRETE PHASES, IS OFTEN EXTREMELY SENSITIVE TO CHANGES IN CONDITIONS AND IMPURITIES; AND
(B) THE COLLOID SYSTEM WHICH IS THE END PRODUCT OF THE POLYMERIZATION AND IS OFTEN UNSTABLE, VARIABLE IN ITS PROPERTIES AND DIFFICULT TO REPRODUCE. And when the solid polymer product is isolated, it characteristically contains, as impurities, small amounts of the various substances introduced to promote the polymerization and stabilize the colloid system. In addition, dispersion and emulsion polymerization processes typically require several to many hours for completion, and are thus distinctly slow processes. In particular, the art discloses (U.S. Pat. No. 3,534,009) that in the batch emulsion polymerization of ethylene and vinyl acetate, an extraordinarily long polymerization time is required to consume all the vinyl acetate monomer charged, and it is necessary to resort to other means to accomplish this objective, such as reduction of ethylene pressure and the addition of more free-radical promoter. In this respect emulsion polymerization processes are at a decided disadvantage relative to the continuous, high pressure bulk polymerization processes now used commercially for the manufacture of low density polyethylene and ethylene copolymers. These latter processes proceed at exceedingly rapid rates; for example in a typical low density polyethylene process, as carried out in a reactor of the type described in U.S. Pat. No. 3,756,996, the polymerization times are of the order of 35 to 45 seconds at a conversion of 12 to 20%, and, the total time required to convert monomer(s) to finished polymer is about 10 minutes.
A further disadvantage of the dispersion and emulsion polymerization processes of the art is that they are as yet of no, or only limited applicability in the manufacture of several important thermoplastic resins of commerce. For example, when ethylene is polymerized in emulsion, polyethylene is produced at typical emulsion polymerization rates (1-7 hours), but it contains residues derived from the emulsifier and a relatively large low molecular weight fraction [G. J. Mantel et al, J. Appl. Polymer Sci., 9, 1797, 1807, (1965); 10, 81,1845 (1966)]. Emulsion polymerization is substantially of no utility in the polymerization of such monomers as propylene, higher alpha-olefins and isobutylene, since these monomers do not produce high polymers by a radical mechanism. Much the same situation holds for the thermoplastic resins produced by polycondensation processes.
It has long been known that many low molecular weight polymers, e.g., various waxes and hydrocarbon resins, can be emulsified in water by first dissolving them in an organic solvent, then contacting the organic solution of the polymer with water in the presence of surface active agents and emulsifiers, and thereafter recovering the organic solvent. This basic process has also been extended to true high polymers. Thus a process is known (U.S. Pat. No. 3,347,811) for preparing aqueous dispersions of ethylene copolymers which comprises (a) dissolving the copolymer in a water-immiscible organic solvent of b.p. 40.degree.-160.degree. C.; (b) emulsifying the solution in a mixture of water and a dissolved surfactant of HLB number of at least 18; and (c) evaporating the organic solvent from the resulting emulsion.
Similarly it is known (U.S. Pat. No. 3,503,917) to prepare artificial latexes, e.g., of polyisobutylene-isoprene copolymer (butyl rubber) and ethylene-propylene rubber, by dissolving the preformed polymers in an organic solvent such as toluene, emulsifying the organic solution of the polymer in water in the presence of a surface active agent, and finally stripping the organic solvent. It is especially to be noted that these processes of the art for dispersion of true high polymers are by their very nature complicated, laborious and, above all, comparatively slow.
In recent years, however, a new simplified process (U.S. Pat. No. 3,422,049; U.S. Pat. No. 3,746,631) has been developed for making dispersions of high molecular weight thermoplastics in water, without the need for an organic solvent. In several important respects the new process differs from all other polymer dispersion processes of the art:
(1) It is a rapid process, requiring a residence time of about 15-20 minutes in its continuous version (U.S. Pat. No. 3,432,483), and operates at 115.degree. C. to 300.degree. C. in the presence of only water and a surfactant, for which reason it is often called the "water process". In view of the short contact time, it is therefore particularly advantageous to couple the water process with the above-mentioned, equally rapid, high pressure process for making polyethylene and ethylene copolymers; when this is done, dispersions of these polymers can be produced from monomers in overall process times of the order of a half-hour or less
(2) However, prior to the present invention, only a very unique class of surfactants, certain block copolymers of ethylene oxide and propylene oxide, could be used to produce the dispersions.
(3) The particles produced are substantially all spherical, very fine, and tend to be of a narrow particle size range; low density polyethylene, for example, is converted to spherical particles having a number-average particle diameter of about 10 microns, and a weight average diameter of about 25 microns.
(4) Since the water-process does not require the use of an organic solvent, it avoids all the disadvantages associated with prior art processes requiring solvents, such as: solvent loss during processing with attendant air pollution; the fire hazard inherent in solvent usage, and the time and energy expended in dissolving a high polymer in a solvent, and in recovering and recycling the solvent. However, when it is desired to make dispersions of even finer particle size, i.e., particles of submicron diameter (U.S. Pat. No. 3,418,265), limited amounts of organic solvent are advantageously added in the water process, but in amounts of only 0.5 to 20 parts per 100 parts of the resin to be dispersed, amounts that are far less than those required to disperse the resin in the other processes of the prior art. Additionally, in another version of the water process (U.S. Pat. No. 3,522,036), limited amounts of a liquid vinyl monomer, e.g., styrene, may also be added to provide stable, film-forming aqueous latices of high molecular weight polyethylene. In still other variants it is possible to produce foamed particles (U.S. Pat. No. 3,472,801) or to incorporate certain colorants (U.S. Pat. No. 3,449,291) and pigments (U.S. Pat. No. 3,675,736) in the particles.
The above described fine powders are, by virtue of their small particle size, their narrow particle size range, and their spherical particle shape, unique states of matter which cannot readily be prepared by conventional procedures known to the art. However, as already pointed out above, a unique nonionic surfactant or dispersing agent is required, i.e., the aforesaid block copolymer of ethylene oxide and propylene oxide more fully described in U.S. Pat. No. 3,422,049 and sold under the trade name of Pluronics by BASF-Wyandotte Corp. Nevertheless, as experience has accumulated in the use of these nonionic dispersants, certain disadvantages have become apparent. The very fine particle size fraction they produce, e.g., the aforesaid 10 micron particle can present problems in certain situations:
(a) fractions comprising particles 10 microns or less in diameter are classified as "respirable dusts" and may present a health hazard if they escape into the work place air;
(b) in addition, these ultrafine fractions can at times cause problems in powder handling equipment because of plugging and blockage.
Although art related to the water-process (U.S. Pat. No. 3,586,654) does disclose that the particles produced by that dispersion process may be reformed into particles that are the same, larger, or smaller in diameter, this conversion involves two operations and it requires the use of such large amounts of Pluronic dispersant as to be relatively disadvantageous from an economic point of view. Consequently, it would be highly desirable to modify the water-process in such a manner as to be able to disperse thermoplastic resins into particles of any desired particle size in one operation, and to achieve this goal by using economically acceptable levels of dispersing agent (preferably up to about 15 pph of resin), i.e. by substituting a simple dispersion system comprised of readily available components and which obviates some, if not all, of the difficulties encountered with the Pluronic dispersants.
Moreover, increasing petroleum prices make it highly desirable, again for economic reasons to eliminate, if possible the requirement for the petroleum-based Pluronic dispersants. Additionally, as disclosed in commonly assigned, copending application, Ser. No. 564,198, dealing with the simultaneous saponification and dispersion of ethylene-vinyl acetate copolymers, it appears that the Pluronic dispersants are quite sensitive to the presence of metallic salts. When the polymer to be dispersed contained 0.1% or more of sodium ion, the copolymer could not be dispersed to a fine particle size until the sodium ion content is reduced to less than 0.1%.
It is known in the art to employ soaps as emulsifying agents in the emulsion copolymerization of monomers such as styrene and butadiene to produce latices of synthetic rubber. It is further known that the dispersed or emulsified synthetic rubber particles may be coagulated by adding salt or salt and acid to the latices; in this way the rubber may be conveniently recovered as a rubbery crumb. Moreover, if desired, the particle agglomeration process may be arrested at an intermediate particle size by adding salt to the latex, and/or by forming a salt in situ by adding an acid and later a base. Rhines (U.S. Pat. No. 2,538,273), for example, shows that in this latter process, the amount of acid and/or salt necessary to increase particle size can be reduced by also adding an alcohol.
The processes of the present invention, however, differ fundamentally from these teachings of the art in the following important respects:
1. They relate to the dispersion of already-formed synthetic resins, principally and advantageously to those selected from a hereinafter described group of synthetic resins that cannot readily or conveniently be made by emulsion or dispersion polymerization of the respective monomer or monomers.
2. They represent dispersion processes wherein both the soap and the salt are simultaneously present during the dispersion process itself. This contrasts with the above-cited processes of the art in which the salt is added as a coagulant after a latex has been made by emulsion polymerization in the presence of a soap.
3. They relate to the dispersion of already-formed synthetic resins that contain 15 to 35 weight percent of a polar comonomer, and which may usually be dispersed by a soap alone although the particle size is generally relatively coarse, and can be significantly reduced if a salt is also present during the dispersion process. In situ soaps provide finer particles than preformed soaps.
4. They represent dispersion processes in which an optimum salt concentration can be discerned, below which the salt has relatively little effect on particle size and above which the dispersion process fails altogether. The optimum salt concentrations are well below salt concentrations of the art normally employed for latex coagulation, where no upper salt concentration ordinarily exists as far as coagulation is concerned.
5. They represent dispersion processes in which variables such as the nature of the resin, i.e., its composition and melt viscosity; the resin solids content, i.e., ratio of resin to water; nature of the soap, i.e., size of the acid residue, the identity of the cation, and whether the soap is made in situ or added preformed; the concentration of the soap; and the concentration of the salt are all interrelated with each other and especially with the dispersion temperature selected. This dynamic system of variables is much more intricate in nature than latex coagulation processes of the art.
Processes for simultaneously dispersing and saponifying ethylene-vinyl acetate (EVA) copolymers to provide particulate hydrolyzed ethylene-vinyl acetate (HEVA) copolymers are known. In German Democratic Republic (DDR) Patent Specification No. 88,404, there is described a process for simultaneously dispersing and saponifying EVA copolymers employing sodium hydroxide or potassium hydroxide as the saponification agent and an alkyl sulfonate, an acyl derivative of N-methyltaurine, a higher fatty acid soap, an alkaryl sulfonate or a nonionic surface-active agent derived from ethylene oxide as the dispersion agent.
The process described involves saponifying ethylenevinyl acetate copolymers at elevated temperature and pressure including, as the final step, discharging the reaction mixture at the operating temperature and pressure directly into a quench vessel at atmospheric or subatmospheric pressure. The quench vessel contains water that is stirred during the discharge operation and the rate of discharge of the reaction mixture is regulated by means of a needle valve. Thus, the sudden release of the reaction mixture causing a portion of the reaction medium to vaporize apparently results in formation of the dispersion due to the atomizing effect of the needle valve. This patent also discloses the optional use of dispersants, but it is apparent from the data provided that such dispersants have only a secondary effect, the primary determinant of dispersion being the discharge of the hot reaction mixture to the quenching bath. From the particle size distribution data provided in the disclosure, it is clear that the presence of dispersing agent seems to favor smaller particles, but is not absolutely essential since comparable dispersions are obtained when dispersing agents are not present in the reaction mixture. There is no indication that a dispersion of the polymer occurs in the reaction mixture prior to discharge when dispersing agents are present but the data provided shows that, on discharge, a dispersion is produced in the presence or absence of dispersing agent. Attempts to obtain dispersions of saponified EVA using N-oleoylsarcosinate as dispersing agent by merely cooling the reaction mixture without the described discharge step of DDR 88,404 have not produced dispersions. Similarly, when arylsulfonate dispersants are employed in lieu of the sarcosinate, no dispersions are obtained when the reaction mixture is cooled. Thus, it must be concluded that dispersion only occurs on discharge.
The dispersed product obtained by the method of DDR 88,404 is of fairly large particle size, the heavy majority of the particles being of diameters greater than 0.125 mm, i.e., usually over 80% of the dispersed particles. In addition, the product is composed of irregular particles, with no spherical particles being observed.
In accordance with the present invention, desirable improvements are achieved by substituting dispersant systems comprising alkali metal soaps higher carboxylic acids in conjunction with certain water-soluble salts for the unique Pluronic dispersing agents of U.S. Pat. Nos. 3,422,049 and 3,476,631.
This invention provides dispersing systems for dispersing high molecular weight copolymers of olefins in water at rapid rates and avoids the need for an organic solvent. The novel dispersing systems thus provided generally yield particles that are larger than those produced by the Pluronic dispersing agents of the art and afford a wider range of particle size than the Pluronic dispersants, without, however, requiring the use of large amounts of dispersant. Additionally, they practically eliminate the at times objectionable ultrafine, 10-micron or less particle fractions present in powders produced by the Pluronic dispersants.
In general, the present process can be carried out substantially as described in the basic water-process patent (U.S. Pat. No. 3,422,049), with the exception that the Pluronic surfactants of that process are replaced with dispersing agents comprising a soap of a higher carboxylic acid and a water-soluble salt.
Thus, in batch operation, the polymer, water (preferably distilled or deionized) and the dispersant system are introduced into a pressure vessel equipped with an external heater, a thermocouple, and a stirrer. The vessel is sealed, heated to 150.degree.-270.degree. C. and held at the selected temperature for a brief period, usually seven minutes, during which time rapid stirring is applied. Thereafter the heater is shut off and the vessel is allowed to cool with stirring, and optionally with externally-applied cooling for convenience. When the temperature of the contents of the vessel has fallen below about 100.degree. C., the product is discharged, optionally diluted with additional deionized or distilled water and allowed to cool to room temperature. The dispersions may, if desired, be used directly in various applications. Filtration of the dispersions normally permits the separation of the polymer particles and soap as a filter cake, and the water-soluble salts pass into the filtrate. EVA copolymer particles containing about 15 to 25 weight percent vinyl acetate can normally be washed free of soap with hot (70.degree.-95.degree. C.) water. However, EVA copolymer particles containing about 25 to 35 weight percent vinyl acetate and tend to coalesce and may require additional treatment. Such treatment may consist of acidification of the soap to release higher carboxylic acid or reaction with an alkaline earth metal salt, e.g., Ca(OH).sub.2, to convert the soap to an alkaline earth metal soap. The protective action of these materials permits easy washing and drying of the particles without coalescence. Such techniques are described in commonly assigned, concurrently filed copending U.S. Patent Application Ser. No. 824,935. The filter cake is washed with cold water, preferably deionized or distilled, to remove contained water-soluble, substantially neutral salt and the washings are combined with the mother liquor. The combined mother liquor and washings contain substantially all of the water-soluble salt, but very little of any soap or higher carboxylic acid if this has been released as a parting by acidification of the soap dispersant. The acid employed for acidification is desirably selected to match the anion of the water-soluble, substantially neutral salt, e.g., hydrochloric acid for sodium chloride. When a metal salt, e.g., calcium hydroxide, is added to convert the higher carboxylic acid soap to an insoluble soap (calcium soap) for a parting, the mother liquor will contain alkali metal hydroxide which can be recycled, for example to make additional in situ soap or simply neutralized to provide more water-soluble salt. Normally a substantial proportion of the water is then removed by distillation and the residue, containing substantially all of the water-soluble salt and alkali (if any) can be recycled.